1. Field of the Invention
This invention concerns silver halide emulsions and photographic materials which contain these emulsions. More precisely, the invention concerns emulsions which contain silver halide crystal grains which have a novel form and silver halide photographic materials in which these emulsions are used, and more precisely, the invention concerns emulsions which contain rod-like or needle-like silver halide crystal grains and silver halide photographic materials in which these emulsions are used.
2. Description of the Prior Art
Silver iodide, silver bromide, silver chloride and crystals consisting of mixtures of these halides are used as silver halides in photographic materials, and crystal forms ranging from so-called regular grains such as cubic, tetradecahedral, octahedral, rhombic dodecahedral, etc., through irregular grains such as tabular grains to grains of undefined form such as spherical grains are well known. Even rod-like grains have been observed. Rod-like grains and tabular grains are often observed in admixture in various proportions with grains of other forms. This is especially so in cases where the grains are formed under conditions such that the silver ion concentration changes during the precipitation and formation of the emulsion grains. Tabular grains are formed with a planar spread in two dimensions with respect to the thickness of the grain while rod-like grains are formed extending linearly in one direction. Tabular grains are known to be advantageous, when used in emulsions, in respect of the light scattering properties of the emulsion layer, the up-take of spectral sensitizing dyes due to their large surface area, and their covering power after development processing. Tabular silver halide grains of this type have been disclosed in U.S. Pat. Nos. 4,434,226, 4,439,520, 4,433,048, 4,386,156, 4,399,215 and 4,400,463, etc.
On the other hand, techniques for the formation of rod-like or needle-like silver halide grains and the characteristics of emulsions which contain crystal grains of this type are virtually unknown. More precisely, techniques in which such grains are formed at a high frequency and emulsions which contain such grains at high frequency are completely unknown.
The formation of needle-like grains has been described by C. R. Berry, S. J. Marino and C. F. Oster in Photographic Science and Engineering, 5, 332 (1961), but according to this work the rate at which grains of this type are formed is no more than about 2%, and the conditions of formation suggest that the formation of these grains is due simply to spiral displacement and no technique for forming such grains at high frequency as been presented to date. The assignment of the twinned crystal planes and crystal planes which are present in needle-like crystals have been described by E. Klein, H. J. Metz and E. Moisar in Photographische Korrespondenz, 99, 99 (1963) and by D. C. Skillman and C. R. Berry in Photographic Science and Engineering, 8, 65 (1964), respectively. The needle-like grains indicated in the former were formed when the twinned crystal plane was the (221) plane and the surface plane was the (111) plane. The needle-like grains indicated in the latter were formed when the twinned crystal planes were the (111) and (411) planes and the surface plane was the (100) plane. However, in these works the former only shows model diagrams of needle-like grains, and on looking at the photographs in the latter publication, only one or a few needle shaped grains can be seen in admixture with large numbers of non-needle-like grains and there is no report in practice of a technique for forming such grains at a high frequency and no emulsions of this type have been proposed.
Emulsions containing tabular silver halide crystal grains of which the aspect ratio defined as the ratio of the edge length to the thickness of the grain is from 1.5:1 to 7:1 and which are bounded by (100) planes, and a method for the preparation of these emulsions have been disclosed in U.S. Pat. No. 4,063,951. According to the definition shown here, essentially rod-like or needle-like grains are included at the upper limit of the claim, but it is clear from the title and the published photographs that the grains which are formed in practice are really tabular grains and certainly not rod-like or needle-like grains. The examples are of emulsions in which the average aspect ratio is 2:1, and even if the grains which have the highest aspect ratio are considered the value is still only 4:1 at the most. Moreover, it is not possible to form rod-like or needle-like silver halide crystal grains of the type disclosed in the present invention using the method disclosed in the aforementioned patent document.
In connection with tabular grains, (100) non-twinned crystal tabular grains have been reported by A. Mignot, E. Francois and M. Catinat in the Journal of Crystal Growth, 23, 207 (1974) where the formation of rod-like grains by spiral displacement has also been described. Photographs are also shown, but the rod-like grains have only been formed in admixture with many grains of other forms and it certainly cannot be said that the rod-like grains are formed at a high frequency, and, moreover, it cannot be said that conditions under which dilute solutions are added over long periods of time are specified for forming such grains at a high frequency and neither can it be said that these conditions are desirable from the point of view of their industrial usefulness. Tabular grains of a similar type have also been disclosed in U.S. Pat. No. 4,386,156 but these grains are completely different from those of the present invention.
Processing speeds of photographic materials have increased in recent years and techniques which enable even more rapid processing to be achieved will be required in the future. The development of techniques for the realization of new levels of performance and applications are also awaited.